Currently, a file transmission has been performed in various methods through an Internet as well as a variety of communication networks including a mobile communication network, and the like.
A parallel download for a file is a method, if a file to be downloaded to a client is present in servers or other clients serving as nodes connected through a communication network, which divides the file into a plurality of blocks and simultaneously requests the download of the blocks from the multiple servers or clients, thus to reduce a download time of the file.
With the reduced download time of a file along with the development of
the communication network, such parallel downloading method for a file has been widely used in a Peer To Peer (P2P), a Grid Computing and the like.
As a related art method for selecting and downloading blocks to perform the parallel downloading method for a file, a sequential selection, a random selection, a rarest first selection, and the like are mainly used.
The sequential selection method is a method for sequentially selecting blocks constituting a file and requesting the download of the blocks from nodes having the blocks.
According to the sequential selection method, if a plurality of nodes are assumed to download the same file, the number of copied blocks at an end of the file on a communication network would be smaller than that of copied blocks at a front thereof until all nodes complete the downloads. Therefore, it would be difficult to download the blocks at the end of the file.
The random selection method is a method for randomly selecting one among blocks constituting a file and requesting the download of the block from nodes having the block.
According to the random selection method, if it assumed that operation rates of all nodes downloading the same file are the same, the number of each copied block would be the same.
However, in an ad-hoc network such as the P2P network, the operation rates of the nodes are different and independent from each other. Accordingly, the random selection method is unable to assure that the copied
numbers of each block for the file being downloaded are the same.
The rarest first selection method is a method for calculating the number of each block belonging to nodes connected through a communication network for a file being downloaded, and requesting a download of a block having the smallest copied number.
The rarest first selection method has an advantage of assuring that the number of the blocks being downloaded is uniformly maintained, however, the operation rates of the nodes having the blocks are different from each other, thereby making it difficult to uniformly maintain a probability to access the nodes having each of the blocks constituting the file.